1. Field of Invention
The present invention relates to phosphorescent or photoluminescent materials and, more particularly, is directed to a non-toxic phosphorescent marking material and a method of marking same.
2. Description of the Prior Art
It has long been recognized that the appeal of certain toys can be enhanced by imparting phosphorescent properties to the various elements thereof. Crayons have become a standard teaching tool for children because they allow children to express themselves artistically. Crayons are clean, safe and easy to use. A need exists for a phosphorescent marker which is safe for children.
Heretofore it has been possible to impart phosphorescent properties to paint and chalk due to their thick viscosity. For example, phosphorescent pigments can be mixed in paint. U.S. Pat. No. 2,396,219 discloses a phosphorescent chalk writing material which is toxic and unsafe for use by children. In addition, chalk makes dust which is not optimal for a permanent clean marking material. The use of a phosphorescent material such as calcium sulphide phosphor in a crayon is not acceptable because calcium sulphide phosphor is toxic and it will settle out. The use of non-toxic phosphorescent pigments in a wax base marking material has been met with limited success because of the lack of a suitable suspension formula and an acceptable suspension process. Currently available phosphorescent pigment particles that are encapsulated in glass frit are heavy and cannot be effectively made into a phosphorescent crayon by conventional methods because the particles settle out of the formula and/or are not distributed throughout the crayon. Autoradiogram marking pens, such as the phosphorescent ink marking pen disclosed in U.S. Pat. No. 4,510,392, are sufficient for exposing film, but they have very low luminous properties for visual effects.
While non-toxic phosphorescent material is available, the pigment is generally unsuitable for use in wax base marking materials because of the problems and difficulties in suspending the phosphorescent material in the base mixture. U.S. Pat. No. 3,392,123 discloses an oxygen activated, wax based, chemiluscent marking material. The marking material is limited in use because it can be activated only once and the tetrakis (demethylamino) ethylene contained therein is corrosive. A need exists for a non-toxic wax base marking material containing suspended phosphorescent particles which can be molded or extruded.